Particulate active material, power storage device positive electrode, power storage device, and production method for particulate active material

ABSTRACT

A particulate active material for a power storage device positive electrode having a higher energy density is provided, which includes particles of an electrically conductive polymer and a conductive agent, wherein the electrically conductive polymer particles each have a surface coated with the conductive agent.

This is a continuation of application Ser. No. 14/441,563, filed May 8,2015, which is a National Stage of International Application No.PCT/JP2013/080488, filed Nov. 12, 2013, claiming priority from JapanesePatent Application No. 2012-249683, filed Nov. 13, 2012, the contents ofwhich are hereby incorporated by reference into this application.

TECHNICAL FIELD

The present invention relates to a particulate active material for apositive electrode of a higher performance power storage device having ahigher energy density, a power storage device positive electrode and apower storage device each employing the particulate active material, anda production method for the particulate active material for the powerstorage device positive electrode.

BACKGROUND ART

With recent improvement and advancement of electronics technology formobile PCs, mobile phones, personal digital assistants (PDAs) and thelike, secondary batteries and the like, which can be repeatedly chargedand discharged, are widely used as power storage devices for theseelectronic apparatuses. For these secondary batteries and otherelectrochemical power storage devices, electrode materials desirablyhave a higher capacity and a rapid charge/discharge property.

An electrode for such a power storage device contains an active materialwhich is capable of ion insertion/desertion. The ion insertion/desertionof the active material is also referred to as doping/dedoping, and thedoping/dedoping amount per unit molecular structure is referred to asdope ratio (or doping ratio). A material having a higher doping ratiocan provide a higher capacity battery.

From an electrochemical viewpoint, the capacity of the battery can beincreased by using an electrode material having a greater ioninsertion/desertion amount. In lithium secondary batteries, which areattractive power storage devices, more specifically, a graphite-basednegative electrode capable of lithium ion insertion/desertion is used inwhich about one lithium ion is inserted and deserted with respect to sixcarbon atoms to provide a higher capacity.

Of these lithium secondary batteries, a lithium secondary battery whichhas a higher energy density and is therefore widely used as the powerstorage device for the aforesaid electronic apparatuses includes apositive electrode prepared by using a lithium-containing transitionmetal oxide such as lithium manganese oxide or lithium cobalt oxide anda negative electrode prepared by using a carbon material capable oflithium ion insertion/desertion, the positive electrode and the negativeelectrode being disposed in opposed relation in an electrolyte solution.

However, this lithium secondary battery, which generates electric energythrough an electrochemical reaction, disadvantageously has a lower powerdensity because of its lower electrochemical reaction rate. Further, thelithium secondary battery has a higher internal resistance, so thatrapid discharge and rapid charge of the secondary battery are difficult.In addition, the secondary battery generally has a shorter service life,i.e., a poorer cycle characteristic, because the electrodes and theelectrolyte solution are degraded due to the electrochemical reactionassociated with the charge and the discharge.

There is also known a lithium secondary battery in which an electricallyconductive polymer, such as a polyaniline containing a dopant, is usedas a positive electrode active material to cope with the aforesaidproblem (see PTL1).

In general, however, the secondary battery employing the electricallyconductive polymer as the positive electrode active material is of ananion migration type in which the electrically conductive polymer isdoped with an anion in a charge period and dedoped with the anion in adischarge period. Where a carbon material or the like capable of lithiumion insertion/desertion is used as a negative electrode active material,it is therefore impossible to provide a rocking chair-type secondarybattery of cation migration type in which a cation migrates between theelectrodes in the charge/discharge. That is, the rocking chair-typesecondary battery is advantageous in that a smaller amount of theelectrolyte solution is required, but the secondary battery employingthe electrically conductive polymer as the positive electrode activematerial cannot enjoy this advantage. Therefore, it is impossible tocontribute to the size reduction of the power storage device.

To cope with this problem, a secondary battery of a cation migrationtype is proposed which is substantially free from change in the ionconcentration of the electrolyte solution without the need for a greateramount of the electrolyte solution, and aims at improving the capacitydensity and the energy density per unit volume or per unit weight. Thissecondary battery includes a positive electrode prepared by using anelectrically conductive polymer containing a polymer anion such aspolyvinyl sulfonate as a dopant, and a negative electrode of metallithium (see PTL2).

RELATED ART DOCUMENT Patent Documents

-   PTL1: JP-A-HEI3(1991)-129679-   PTL2: JP-A-HEI1(1989)-132052

SUMMARY OF INVENTION

However, the secondary batteries described above are not satisfactory inperformance. That is, these batteries are lower in capacity density andenergy density than the lithium secondary battery employing thelithium-containing transition metal oxide such as lithium manganeseoxide or lithium cobalt oxide for the positive electrode.

In order to solve the aforementioned problems associated with the priorart power storage devices such as the lithium secondary batteries, thepresent invention provides a particulate active material for a powerstorage device positive electrode having a higher energy density, thepower storage device positive electrode and a power storage deviceemploying the particulate active material, and a production method forthe active material for the power storage device positive electrode.

The inventors of the present invention conducted intensive studies toprovide a higher performance power storage device having a higher energydensity. The inventors focused on a particulate active material to beused for the power storage device positive electrode in the studies, andfurther conducted studies on the active material. The energy density canbe increased by increasing the proportion of a conductive agent withrespect to the electrically conductive polymer. If the conductive agentis blended in a higher proportion with the electrically conductivepolymer, it is difficult to knead the resulting electrode material. Theinventors further conducted experiments and, as a result, found that ahigher performance power storage device having a higher energy densitycan be provided without blending the conductive agent in a higherproportion by using a particulate active material prepared by coatingsurfaces of particles of the electrically conductive polymer with theconductive agent. Although the reason for this is not necessarilyclarified, this is supposedly because the electrical conductivity of thewhole particulate active material is increased by forming coating layersof the conductive agent on the surfaces of the electrically conductivepolymer particles serving as cores, thereby facilitating the migrationof electrons to a current collector. As a result, ion migration can beefficiently achieved in the charge and discharge, thereby increasing theenergy density.

According to a first inventive aspect, there is provided a particulateactive material for a power storage device positive electrode, theparticulate active material including particles of an electricallyconductive polymer and a conductive agent, wherein the electricallyconductive polymer particles each have a surface coated with theconductive agent.

According to a second inventive aspect, there is provided a powerstorage device positive electrode employing a particulate activematerial including particles of an electrically conductive polymer and aconductive agent, wherein the particles of the electrically conductivepolymer each have a surface coated with the conductive agent.

According to a third inventive aspect, there is provided a power storagedevice, which includes an electrolyte layer, and a positive electrodeand a negative electrode provided in opposed relation with theelectrolyte layer interposed therebetween, wherein the positiveelectrode is produced by using a particulate active material includingparticles of an electrically conductive polymer each having a surfacecoated with a conductive agent.

According to a fourth inventive aspect, there is provided a method forproducing a particulate active material including particles of anelectrically conductive polymer and a conductive agent for a powerstorage device positive electrode, the method including the step ofshearing the particles of the electrically conductive polymer and theconductive agent by means of a composite particle producing apparatus tocoat surfaces of the electrically conductive polymer particles with theconductive agent.

The inventive particulate active material includes the electricallyconductive polymer particles each having a surface coated with theconductive agent and, therefore, the power storage device positiveelectrode containing the particulate active material and the powerstorage device employing the power storage device positive electrodeeach have a higher energy density.

Where the electrically conductive polymer is a polyaniline or apolyaniline derivative, the resulting power storage device is furtherimproved in energy density.

Where the electrically conductive polymer particles and the conductiveagent are sheared by means of the composite particle producingapparatus, the surfaces of the electrically conductive polymer particlescan be uniformly and tightly coated with the conductive agent. Thisincreases the electrical conductivity of the particulate activematerial, thereby further improving the energy density.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing an exemplary power storage device.

FIGS. 2A, 2B and 2C are SEM photographs of electrically conductivepolymer particles of Comparative Example 1, Example 1 and Example 2,respectively.

FIGS. 3A and 3B are TEM photographs of sections of positive electrodesof Comparative Example 1 and Example 1, respectively.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will hereinafter be described indetail by way of example but not by way of limitation.

A particulate active material to be used for a power storage devicepositive electrode (hereinafter sometimes referred to simply as“particulate active material”) according to the present inventionincludes particles of an electrically conductive polymer and aconductive agent, and the electrically conductive polymer particles eachhave a surface coated with the conductive agent. Unless otherwisespecified, the inventive particulate active material means coatedparticles prepared by forming a coating of the conductive agent on thesurfaces of the electrically conductive polymer particles serving ascores.

The power storage device includes, for example, an electrolyte layer 3,and a positive electrode 2 and a negative electrode 4 provided inopposed relation with the electrolyte layer 3 interposed therebetween asshown in FIG. 1, and the inventive particulate active material is usedfor the positive electrode 2. In FIG. 1, reference numerals 1 and 5designate a positive electrode current collector and a negativeelectrode current collector, respectively.

The positive electrode, the negative electrode and the electrolyte layerwill be successively described.

<Positive Electrode>

The positive electrode is produced by using a positive electrodematerial containing the particulate active material including theelectrically conductive polymer particles each having a surface coatedwith the conductive agent.

[Electrically Conductive Polymer]

The electrically conductive polymer serving as the cores of theinventive particulate active material will be described. Theelectrically conductive polymer described above is herein defined as anyof polymers which have an electrical conductivity variable due toinsertion or desertion of ion species with respect to the polymer inorder to compensate for change in electric charge to be generated orremoved by an oxidation reaction or a reduction reaction occurring in amain chain of the polymer.

The polymer has a higher electrical conductivity in a doped state, andhas a lower electrical conductivity in a dedoped state. Even if theelectrically conductive polymer loses its electrical conductivity due tothe oxidation reaction or the reduction reaction to be therebyelectrically insulative (in the dedoped state), the polymer canreversibly have an electrical conductivity again due to theoxidation/reduction reaction. Therefore, the electrically insulativepolymer in the dedoped state is herein also classified into the categoryof the electrically conductive polymer.

A preferred example of the electrically conductive polymer is a polymercontaining a dopant of at least one protonic acid anion selected fromthe group consisting of inorganic acid anions, aliphatic sulfonateanions, aromatic sulfonate anions, polymeric sulfonate anions andpolyvinyl sulfate anions. Another preferred example of the electricallyconductive polymer is a polymer obtained in the dedoped state bydedoping the electrically conductive polymer described above.

Specific preferable examples of the electrically conductive polymerinclude polyacetylene, polypyrrole, polyaniline, polythiophene,polyfuran, polyselenophene, polyisothianaphthene, polyphenylene sulfide,polyphenylene oxide, polyazulene, poly(3,4-ethylenedioxythiophene) andsubstitution products of these polymers. Particularly, polyaniline,polyaniline derivatives, polypyrrole and polypyrrole derivatives eachhaving a higher electrochemical capacity are preferably used, andpolyaniline and polyaniline derivatives are further preferably used.

In the present invention, polyaniline is a polymer prepared byelectrolytic polymerization or chemical oxidation polymerization ofaniline, and the polyaniline derivatives are polymers prepared byelectrolytic polymerization or chemical oxidation polymerization ofaniline derivatives.

Examples of the aniline derivatives include aniline derivatives preparedby substituting aniline at positions other than the 4-position thereofwith at least one substituent selected from the group consisting ofalkyl groups, alkenyl groups, alkoxy groups, aryl groups, aryloxygroups, alkylaryl groups, arylalkyl groups and alkoxyalkyl groups.Specific examples of the aniline derivatives include o-substitutedanilines such as o-methylaniline, o-ethylaniline, o-phenylaniline,o-methoxyaniline and o-ethoxyaniline, and m-substituted anilines such asm-methylaniline, m-ethylaniline, m-methoxyaniline, m-ethoxyaniline andm-phenylaniline, which may be used either alone or in combination.Though having a substituent at the 4-position, p-phenylaminoaniline isadvantageously used as the aniline derivative because polyaniline can beprovided by the oxidation polymerization of p-phenylaminoaniline.

“Aniline or an aniline derivative” is herein referred to simply as“aniline” unless otherwise specified. “At least one of the polyanilineand the polyaniline derivative” is herein referred to simply as“polyaniline” unless otherwise specified. Even if a polymer for theelectrically conductive polymer is prepared from an aniline derivative,therefore, the resulting polymer is referred to as “electricallyconductive polyaniline.”

[Conductive Agent]

In the present invention, the conductive agent to be used for coatingthe surfaces of the electrically conductive polymer particles serving asthe cores of the particulate active material may be an electricallyconductive material free from change in its properties which mayotherwise occur when a potential is applied in the discharge of thepower storage device. Examples of the conductive agent includeelectrically conductive carbon materials and metal materials, amongwhich electrically conductive carbon blacks such as acetylene black andKetjen black, and fibrous carbon materials such as carbon fibers andcarbon nanotubes are preferred. The electrically conductive carbonblacks are particularly preferred.

The proportion of the conductive agent is preferably 1 to 30 parts byweight, more preferably 4 to 20 parts by weight, particularly preferably8 to 18 parts by weight, based on 100 parts by weight of theelectrically conductive polymer. Where the proportion of the conductiveagent is within this range, it is possible to prepare the activematerial without any defect in shape and properties and to effectivelyimprove the rate characteristics.

[Particulate Active Material]

The inventive particulate active material is prepared by shearing theelectrically conductive polymer particles and the conductive agent bymeans of a composite particle producing apparatus. Examples of thecomposite particle producing apparatus include NOBILTA and MECHANOFUSIONavailable from Hosokawa Micron Corporation, MIRROR-D available from NaraMachinery Co., Ltd., and COMPOSI and CONPIX available from Nippon Coke &Engineering Co., Ltd.

The inventive particulate active material thus prepared (coatedparticles prepared by the coating with the conductive agent) preferablyhas a particle size (median diameter) of 0.001 to 1000 μm, morepreferably 0.01 to 100 μm, particularly preferably 0.1 to 20 μm. Themedian diameter is measured, for example, by means of a staticlight-scattering particle diameter distribution analyzer or the like.

The electrically conductive polymer particles before the coating withthe conductive agent have substantially the same particle size as theparticulate active material (coated particles).

In the present invention, a binder, a conductive agent, water and thelike may be blended as required with the particulate active material forthe positive electrode material.

Usable examples of the binder include vinylidene fluoride and astyrene-butadiene rubber. Other examples of the binder include a polymeranion, anion compounds having a relatively great molecular weight, andanionic polymers having a lower solubility in the electrolyte solution.

Particularly, the anionic polymer is preferably used as a majorcomponent of the binder. The major component herein means a componentthat accounts for the majority of the binder, and the binder may includethe major component alone.

Examples of the anionic polymer materials include a polymer anion,anionic compounds each having a relatively great molecular weight, andanionic compounds each having a lower solubility in the electrolytesolution. More specifically, a compound having a carboxyl group in amolecule thereof is preferably used and, particularly, a polymericpolycarboxylic acid is preferably used. Where the polycarboxylic acid isused as the anionic polymer material, the polycarboxylic acid functionsas the dopant in addition to the binder, thereby improving thecharacteristic properties of the power storage device.

Examples of the polycarboxylic acid include polyacrylic acid,polymethacrylic acid, polyvinylbenzoic acid, polyallylbenzoic acid,polymethallylbenzoic acid, polymaleic acid, polyfumaric acid,polyglutamic acid and polyasparaginic acid, among which polyacrylic acidand polymethacrylic acid are particularly preferred. Thesepolycarboxylic acids may be used either alone or in combination.

The polycarboxylic acid may be a polycarboxylic acid oflithium-exchanged type prepared by lithium-exchanging carboxyl groups ofa carboxyl-containing compound. The lithium exchange percentage isideally 100%, but may be lower, preferably 40% to 100%.

Where the polymer such as the polycarboxylic acid is used as the binder,the polymer also functions as the dopant. Therefore, the inventive powerstorage device has a rocking chair-type mechanism, which contributes tothe improvement of the characteristic properties of the power storagedevice.

The binder is generally used in an amount of 1 to 100 parts by weight,preferably 2 to 70 parts by weight, most preferably 5 to 40 parts byweight, based on 100 parts by weight of the electrically conductivepolymer. If the amount of the binder is excessively small, it will beimpossible to provide a homogenous electrode. If the amount of thebinder is excessively great, the relative amount of the active materialis reduced, making it impossible to provide a power storage devicehaving a higher energy density.

In the present invention, the conductive agent to be optionally blendedin the positive electrode material as required may be the same as theconductive agent to be used for the particulate active material. Theconductive agent may be an electrically conductive material free fromchange in its properties which may otherwise occur when a potential isapplied in the discharge of the power storage device. Examples of theconductive agent include electrically conductive carbon materials andmetal materials, among which electrically conductive carbon blacks suchas acetylene black and Ketjen black, and fibrous carbon materials suchas carbon fibers and carbon nanotubes are preferred. The electricallyconductive carbon blacks are particularly preferred.

The conductive agent to be optionally used is generally separate fromthe conductive agent to be used for the coating of the surfaces of theelectrically conductive polymer particles, but may be the same as ordifferent from the conductive agent for the particulate active material.

The proportion of the optional conductive agent is preferably 1 to 30parts by weight, more preferably 4 to 20 parts by weight, particularlypreferably 8 to 18 parts by weight, based on 100 parts by weight of theelectrically conductive polymer.

The inventive power storage device positive electrode is preferably madeof a composite material including the particulate active material, thebinder and the like, and is generally provided in the form of poroussheet.

The positive electrode typically has a thickness of 1 to 500 μm, morepreferably 10 to 300 μm. The thickness of the positive electrode ismeasured, for example, by means of a dial gage (available from OzakiMfg. Co., Ltd.) which is a flat plate including a distal portion havinga diameter of 5 mm. The measurement is performed at ten points on asurface of the electrode, and the measurement values are averaged. Wherethe positive electrode (porous layer) is provided on the currentcollector and combined with the current collector, the thickness of thecombined product is measured in the aforementioned manner, and themeasurement values are averaged. Then, the thickness of the positiveelectrode is determined by subtracting the thickness of the currentcollector from the average thickness of the combined product.

The inventive power storage device positive electrode is produced, forexample, in the following manner. The conductive agent, the binder andwater are added to the particulate active material including theelectrically conductive polymer particles, and a slurry is prepared bydispersing the particulate active material in the resulting mixture.Then, the slurry is applied onto the current collector, and shaped intoa sheet by evaporating water from the slurry. Thus, the positiveelectrode (sheet electrode) is provided as a composite product in whicha layer of a mixture of the particulate active material and the optionalbinder is provided on the current collector.

<Negative Electrode>

The negative electrode described above is preferably produced from ametal or a negative electrode material (negative electrode activematerial) capable of ion insertion/desertion. Examples of the negativeelectrode active material include metal lithium, carbon materials andtransition metal oxides capable of insertion and desertion of lithiumions in oxidation and reduction, silicon and tin. The negative electrodepreferably has substantially the same thickness as the positiveelectrode.

<Electrolyte Layer>

The electrolyte layer described above is formed from an electrolyte. Forexample, a sheet including a separator impregnated with an electrolytesolution or a sheet made of a solid electrolyte is preferably used. Thesheet made of the solid electrolyte per se functions as a separator.

The electrolyte includes a solute and, as required, a solvent andadditives. Preferred examples of the solute include compounds preparedby combining a metal ion such as a lithium ion with a proper counter ionsuch as a sulfonate ion, a perchlorate ion, a tetrafluoroborate ion, ahexafluorophosphate ion, a hexafluoroarsenate ion, abis(trifluoromethanesulfonyl)imide ion, abis(pentafluoroethanesulfonyl)imide ion or a halide ion. Specificexamples of the electrolyte include LiCF₃SO₃, LiClO₄, LiBF₄, LiPF₆,LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂ and LiCl.

Examples of the solvent include nonaqueous solvents, i.e., organicsolvents, such as carbonates, nitriles, amides and ethers. Specificexamples of the organic solvents include ethylene carbonate, propylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,ethyl methyl carbonate, acetonitrile, propionitrile,N,N′-dimethylacetamide, N-methyl-2-pyrrolidone, dimethoxyethane,diethoxyethane and γ-butyrolactone, which may be used either alone or incombination. A solution prepared by dissolving the solute in the solventmay be referred to as “electrolyte solution.”

Examples of the additives include an electrode surface controlling agentsuch as vinylene carbonate, an overcharge preventing agent such asbiphenyl, cyclohexylbenzene and fluorinated anisole, a flame retardantsuch as phosphates and phosphazenes, which comprehensively balance theoperation voltage, the charge/discharge rate, the safety, the servicelife and the like.

The power storage device includes a separator in addition to the currentcollectors, the positive electrode, the electrolyte layer and thenegative electrode. The separator may be used in a variety of forms. Forexample, the separator may be an insulative porous sheet which iscapable of preventing an electrical short circuit between the positiveelectrode and the negative electrode and electrochemically stable andhas a higher ion permeability and a certain mechanical strength.Preferably usable examples of the porous sheet include paper, nonwovenfabrics, porous sheets made of resins such as polypropylene,polyethylene and polyimide, which may be used either alone or incombination. Where the electrolyte layer 3 is a solid electrolyte sheetas described above, the solid electrolyte sheet per se functions as theseparator, thereby obviating the need for additionally preparing theseparator.

The current collectors 1, 5 shown in FIG. 1 are required to have ahigher electron conductivity and permit volume reduction in the insideof the battery (thickness reduction) and easy processing. Exemplarymaterials for the current collectors satisfying these characteristicrequirements include metal foils and meshes such as of nickel, aluminum,stainless steel and copper. The positive electrode current collector(e.g., current collector 1) and the negative electrode current collector(e.g., current collector 5) may be formed of the same material or may beformed of different materials.

<Power Storage Device>

Next, the power storage device employing the inventive power storagedevice positive electrode will be described. The inventive power storagedevice includes, for example, the electrolyte layer 3, and the positiveelectrode 2 and the negative electrode 4 provided in opposed relationwith the electrolyte layer 3 interposed therebetween as shown in FIG. 1.

The power storage device employing the inventive power storage devicepositive electrode is produced, for example, in the following manner byusing the aforementioned negative electrode and the like. That is, thepositive electrode, the separator and the negative electrode are stackedwith the separator interposed between the positive electrode and thenegative electrode, whereby a stacked component is prepared. The stackedcomponent is put in a battery container such as an aluminum laminatepackage, and then the resulting battery container is dried in vacuum. Inturn, an electrolyte solution is injected in the battery container driedin vacuum, and an opening of the package (battery container) is sealed.Thus, the power storage device is produced. The battery productionprocess including the injection of the electrolyte solution in thepackage is preferably performed in a glove box in an inert gasatmosphere such as an ultrapure argon gas atmosphere.

Besides the laminate cell, the inventive power storage device may beprovided in a variety of forms including a film form, a sheet form, asquare form, a cylindrical form and a button form. In the case of thelaminate cell, the positive electrode of the power storage devicepreferably has an edge length of 1 to 300 mm, particularly preferably 10to 50 mm, and the negative electrode preferably has an edge length of 1to 400 mm, particularly preferably 10 to 60 mm. The negative electrodepreferably has a slightly greater size than the positive electrode.

The inventive power storage device, like an electric double layercapacitor, has a higher weight power density and excellent cyclecharacteristics. In addition, the power storage device has asignificantly higher weight energy density than the prior art electricdouble layer capacitor. Therefore, the power storage device may be akind of a capacitor-type power storage device.

EXAMPLES

Inventive examples will hereinafter be described in conjunction withcomparative examples. However, the prevent invention is not limited tothese examples.

The following components were prepared before the production of powerstorage devices according to the inventive examples and the comparativeexamples.

<Preparation of Electrically Conductive Polyaniline Powder>

Powder of an electrically conductive polyaniline (electricallyconductive polymer) containing tetrafluoroboric acid as a dopant wasprepared in the following manner. That is, 84.0 g (0.402 mol) of atetrafluoroboric acid aqueous solution (special grade reagent availablefrom Wako Pure Chemical Industries, Ltd.) having a concentration of 42wt % was added to 138 g of ion-exchanged water contained in a 300-mLvolume glass beaker. Then, 10.0 g (0.107 mol) of aniline was added tothe resulting solution, while the solution was stirred by a magneticstirrer. Immediately after the addition of aniline to thetetrafluoroboric acid aqueous solution, aniline was dispersed in an oilydroplet form in the tetrafluoroboric acid aqueous solution, and thendissolved in water in several minutes to provide a homogeneoustransparent aniline aqueous solution. The aniline aqueous solution thusprovided was cooled to −4° C. or lower with the use of a refrigerantincubator.

Then, 11.63 g (0.134 mol) of a powdery manganese dioxide oxidizing agent(Grade-1 reagent available from Wako Pure Chemical Industries, Ltd.) wasadded little by little to the aniline aqueous solution, while themixture in the beaker was kept at a temperature of not higher than −1°C. Immediately after the oxidizing agent was thus added to the anilineaqueous solution, the color of the aniline aqueous solution turned darkgreen. Thereafter, the solution was continuously stirred, wherebygeneration of a dark green solid began.

After the oxidizing agent was added in 80 minutes in this manner, theresulting reaction mixture containing the reaction product thusgenerated was cooled, and further stirred for 100 minutes. Thereafter,the resulting solid was suction-filtered through No. 2 filter paper(available from ADVANTEC Corporation) with the use of a Buchner funneland a suction bottle to provide powder. The powder was washed in anabout 2 mol/L tetrafluoroboric acid aqueous solution with stirring bymeans of the magnetic stirrer, then washed in acetone several times withstirring, and suction-filtered. The resulting powder was dried in vacuumat a room temperature (25° C.) for 10 hours. Thus, 12.5 g of anelectrically conductive polyaniline containing tetrafluoroboric acid asa dopant (hereinafter referred to simply as “electrically conductivepolyaniline”) was provided, which was bright green powder.

(Electrical Conductivity of Electrically Conductive Polyaniline Powder)

After 130 mg of the electrically conductive polyaniline powder wasmilled in an agate mortar, the resulting powder was compacted into anelectrically conductive polyaniline disk having a diameter of 13 mm anda thickness of 720 μm in vacuum at a pressure of 75 MPa for 10 minutesby means of a KBr tablet forming machine for infrared spectrummeasurement. The disk had an electrical conductivity of 19.5 S/cmmeasured by four-point electrical conductivity measurement by the Vander Pauw method.

(Preparation of Electrically Conductive Polyaniline Powder in DedopedState)

The electrically conductive polyaniline powder prepared in the dopedstate in the aforementioned manner was put in a 2 mol/L sodium hydroxideaqueous solution, and stirred in a 3-L separable flask for 30 minutes.Thus, the electrically conductive polyaniline powder was dedoped withthe tetrafluoroboric acid dopant through a neutralization reaction. Thededoped polyaniline was washed with water until the filtrate becameneutral. Then, the dedoped polyaniline was washed in acetone withstirring, and suction-filtered through No. 2 filter paper with the useof a Buchner funnel and a suction bottle. Thus, dedoped polyanilinepowder was provided on the No. 2 filter paper. The resulting powder wasdried in vacuum at a room temperature for 10 hours, whereby browndedoped polyaniline powder was provided.

(Preparation of Polyaniline Powder in Reduced-Dedoped State)

Next, the dedoped polyaniline powder was put in a phenylhydrazinemethanol aqueous solution, and reduced for 30 minutes with stirring. Dueto the reduction, the color of the polyaniline powder turned from brownto gray. After the reaction, the resulting polyaniline powder was washedwith methanol and then with acetone, filtered, and dried in vacuum at aroom temperature. Thus, reduced-dedoped polyaniline was provided.

A particle of the resulting powder had a median diameter of 13 μm asmeasured by a light scattering method by using acetone as a solvent.

(Electrical Conductivity of Reduced-Dedoped Polyaniline Powder)

After 130 mg of the reduced-dedoped polyaniline powder was milled in anagate mortar, the resulting powder was compacted into areduced-dedopedpolyaniline disk having a thickness of 720 μm in vacuumat a pressure of 75 MPa for 10 minutes by means of a KBr tablet formingmachine for infrared spectrum measurement. The disk had an electricalconductivity of 5.8×10⁻³ S/cm measured by four-point electricalconductivity measurement by the Van der Pauw method. This means that thepolyaniline compound was an active material compound having anelectrical conductivity variable due to ion insertion/desertion.

<Preparation of Binder Solution>

Polyacrylic acid (available from Wako Pure Chemical Industries, Ltd.,and having a weight average molecular weight of 1,000,000) was dissolvedin water, whereby 20.5 g of a homogeneously viscous polyacrylic acidaqueous solution having a concentration of 4.4 wt % was provided. Then,0.15 g of lithium hydroxide was added to and dissolved in the resultingpolyacrylic acid aqueous solution, whereby a polyacrylic acid-lithiumpolyacrylate composite solution (binder solution) was prepared in which50% of acrylic acid portions were lithium-exchanged.

<Preparation of Separator>

A nonwoven fabric (TF40-50 available from Hohsen Corporation and havinga porosity of 55%) was prepared.

<Preparation of Negative Electrode>

Metal lithium (rolled metal lithium available from Honjo Metal Co.,Ltd.) having a thickness of 50 μm was prepared.

<Preparation of Electrolyte Solution>

An ethylene carbonate/dimethyl carbonate solution containing lithiumtetrafluoroborate (LiBF₄) at a concentration of 1 mol/dm³ (availablefrom Kishida Chemical Co., Ltd.) was prepared.

<Tab Electrodes>

A 50-μm thick aluminum metal foil was prepared as a current extractiontab electrode for the positive electrode, and a 50-μm thick nickel metalfoil was prepared as a current extraction tab electrode for the negativeelectrode.

<Current Collectors>

A 30-μm thick aluminum foil was prepared as a positive electrode currentcollector, and a 180-μm thick stainless steel mesh was prepared as anegative electrode current collector.

Positive electrode slurries were each prepared for production of thepositive electrode by using the materials prepared in the aforementionedmanner.

[Slurry for Example 1]

First, 4 g of the polyaniline powder prepared in the aforementionedmanner and 0.5 g (equivalent to 13 parts by weight based on 100 parts byweight of the polyaniline powder) of electrically conductive carbonblack (conductive agent, DENKA BLACK available from Denki Kagaku KogyoK.K.) were treated by means of a composite particle producing apparatus(NOBILTA available from Hosokawa Micron Corporation) for 30 minutesunder rotational conditions with a load power of 500 W in a volume of 80cc. Thus, polyaniline particles each having a surface coated with theconductive agent were produced. After the resulting polyanilineparticles were added to 20.5 g of the polyacrylic acid/lithiumpolyacrylate composite solution prepared in the aforementioned mannerand thoroughly kneaded by a spatula, the resulting mixture wasultrasonically treated for 5 minutes by an ultrasonic homogenizer, andtreated by a thin-film spin type high-speed mixer (FILMIX MODEL 40-40available from Primix Corporation). Thus, a fluid slurry was prepared.The slurry was defoamed for 3 minutes by means of a planetary mixer(THINKY MIXER available from Thinky Corporation).

[Slurry for Example 2]

A slurry was prepared in substantially the same manner as in Example 1,except that the amount of the electrically conductive carbon black(conductive agent, DENKA BLACK available from Denki Kagaku Kogyo K.K.)was increased to 1.0 g.

[Slurry for Comparative Example 1]

A slurry was prepared in substantially the same manner as in Example 1,except that the treatment for the coating with the conductive agent bymeans of the composite particle producing apparatus (NOBILTA availablefrom Hosokawa Micron Corporation) was not performed. More specifically,after 4 g of the polyaniline powder prepared in the aforementionedmanner, 0.5 g of electrically conductive carbon black (conductive agent,DENKA BLACK available from Denki Kagaku Kogyo K.K.) and 4 g of waterwere mixed together, the resulting mixture was added to 20.5 g of thepolyacrylic acid/lithium polyacrylate composite solution prepared in theaforementioned manner, and thoroughly kneaded by a spatula. Then, theresulting mixture was ultrasonically treated for 5 minutes by anultrasonic homogenizer, and treated by a thin-film spin type high-speedmixer (FILMIX MODEL 40-40 available from Primix Corporation). Thus, afluid slurry was prepared. The slurry was defoamed for 3 minutes bymeans of a planetary mixer (THINKY MIXER available from ThinkyCorporation). Thus, a slurry of Comparative Example 1 was prepared.

Examples 1 and 2

The slurries prepared in the aforementioned manner for Examples 1 and 2were each applied at a coating rate of 10 mm/sec onto an etched aluminumfoil for an electric double layer capacitor (30CB available from HohsenCorporation) with the use of a desktop automatic coater (available fromTester Sangyo Co., Ltd.) while the coating thickness was adjusted to 360μm by a doctor blade applicator equipped with a micrometer. Theresulting coatings were allowed to stand at a room temperature (25° C.)for 45 minutes, and then dried on a hot plate kept at a temperature of100° C. Thus, polyaniline sheet electrodes (positive electrodes) wereproduced.

Comparative Example 1

The slurry prepared in the aforementioned manner for Comparative Example1 was applied at a coating rate of 10 mm/sec onto an etched aluminumfoil for an electric double layer capacitor (30CB available from HohsenCorporation) with the use of a desktop automatic coater (available fromTester Sangyo Co., Ltd.) while the coating thickness was adjusted to 360μm by a doctor blade applicator equipped with a micrometer. Then, theresulting coating was dried in a dryer kept at a temperature of 150° C.for 20 minutes. Thus, a polyaniline sheet electrode (positive electrode)was produced.

The states of the coatings of the conductive agent in the respectiveparticulate active materials were each observed at a magnification of5000 by means of a scanning electron microscope (SEM S3500N availablefrom HITACHI Ltd.) FIGS. 2A, 2B and 2C are SEM photographs of theparticulate active material used in Comparative Example 1, Example 1 andExample 2, respectively.

The particulate active material of Comparative Example 1 shown in FIG.2A was prepared without the addition of the conductive agent and,therefore, was the original electrically conductive polymer particles,which each had a surface not coated with the conductive agent. Incontrast, the particulate active materials of Examples 1 and 2 shown inFIGS. 2B and 2C, respectively, were free from significant change inparticle shape, though each having a slightly greater size than theoriginal electrically conductive polymer particles.

Further, the sections of the positive electrodes of Example 1 andComparative Example 1 were observed through TEM measurement. After thepositive electrodes were impregnated with an impregnation resin to fillinternal voids thereof with the impregnation resin, the positiveelectrodes were each sliced by an ultra-thin slicing method, and theresulting thin slices were observed at a magnification of 5000 by meansof a transmissive electron microscope (TEM H-7650 available from HitachiHigh-Technologies Corporation). The results are shown in FIGS. 3A and3B. FIGS. 3A and 3B are TEM photographs of Comparative Example 1 andExample 1, respectively.

In Comparative Example 1, foggy minute spherical particles observed inFIG. 3A were the conductive agent (carbon black), which were presentseparately from the electrically conductive polymer particles. InExample 1, in contrast, foggy minute spherical particles of theconductive agent were not observed in FIG. 3B, but the particulateactive material had a slightly greater particle size than the originalelectrically conductive polymer particles. This indicates that theconductive agent adhered to the surfaces of the electrically conductivepolymer particles. Thus, FIGS. 2A to 2C and FIGS. 3A and 3B indicatethat the electrically conductive polymer particles of the particulateactive materials of Examples 1 and 2 each had a surface coated with theconductive agent.

<Production of Power Storage Device>

Laminate cells serving as power storage devices (lithium secondarybatteries) were each assembled in the following manner by employing thepositive electrodes (polyaniline sheet electrodes) produced in Examples1 and 2 and Comparative Example 1 and other materials prepared asdescribed above.

A battery assembling process was performed in a glove box in anultrapure argon gas atmosphere (having a dew point of −100° C. therein).

The positive electrode for the laminate cell had an electrode size of 27mm×27 mm, and the negative electrode had an electrode size of 29 mm×29mm, which was slightly greater than the positive electrode size.

For use, the metal foils of the tab electrodes for the positiveelectrode and the negative electrode were preliminarily connected to thecorresponding current collectors by means of a spot welding machine. Thepolyaniline sheet electrode (positive electrode), the stainless steelmesh prepared as the negative electrode current collector, and theseparator were dried in vacuum at 100° C. for 5 hours. Thereafter, thesematerials were put in the glove box having a dew point of −100° C. Then,the prepared metal lithium foil was pressed against and squeezed intothe stainless steel mesh of the current collector in the glove box,whereby a negative electrode/current collector assembly was produced.

Subsequently, the separator was held between the positive electrode andthe negative electrode, and the resulting assembly was put in a laminatepack having three heat-sealed sides. The position of the separator wasadjusted so that the positive electrode and the negative electrode wereproperly opposed to each other without short circuit. Then, a sealantwas applied on the positive electrode tab and the negative electrodetab, and the tab electrode portions were heat-sealed with an electrolytesolution inlet port kept open. Thereafter, a predetermined amount of anelectrolyte solution was sucked into a micropipette, and fed into thelaminate pack through the electrolyte solution inlet port. Finally, theelectrolyte solution inlet port provided at an upper portion of thelaminate pack was heat-sealed, whereby the laminate cell was completed.

The following characteristic property of each of the laminate cells(power storage devices) thus produced were measured in the followingmanner. The results are shown below in Table 1.

<Measurement of Energy Density (mWh/g)>

The energy density of each of the power storage devices was measured ina constant current and constant voltage charge/constant currentdischarge mode at 25° C. by means of a battery charge/discharge device(SD8 available from Hokuto Denko Corporation). The charge terminationvoltage was set to 3.8 V. After the voltage reached 3.8 V through aconstant current charge process, a constant voltage charge process wasfurther performed at 3.8 V for 2 minutes. Thereafter, a constant currentdischarge process was performed to a discharge termination voltage of2.0 V. Provided that the polyaniline had a weight capacity density of150 mAh/g, the overall capacity density (mAh/g) was calculated based onthe amount of polyaniline contained in a unit area of the electrode ofthe power storage device. The energy density was determined so that thepower storage device was charged to the full capacity and discharged in20 hours (0.05 C).

TABLE 1 Example Example Comparative 1 2 Example 1 Energy density (mWh/g)506 532 455

The results shown in Table 1 indicate that the power storage devices ofExamples 1 and 2, which each employed the positive electrode containingthe electrically conductive polymer particles each having a surfacecoated with the conductive agent, each had a higher energy density thanthe power storage device of Comparative Example 1, which employed thepositive electrode containing the electrically conductive polymerparticles each having a surface not coated with the conductive agent.

While specific forms of the embodiment of the present invention havebeen shown in the aforementioned inventive examples, the inventiveexamples are merely illustrative of the invention but not limitative ofthe invention. It is contemplated that various modifications apparent tothose skilled in the art could be made within the scope of theinvention.

The inventive power storage device can be advantageously used as alithium secondary battery and other power storage devices. The inventivepower storage device can be used for the same applications as the priorart secondary batteries, for example, for mobile electronic apparatusessuch as mobile PCs, mobile phones and personal digital assistants(PDAs), and for driving power sources for hybrid electric cars, electriccars and fuel battery cars.

REFERENCE SIGNS LIST

-   1 CURRENT COLLECTOR (FOR POSITIVE ELECTRODE)-   2 POSITIVE ELECTRODE-   3 ELECTROLYTE LAYER-   4 NEGATIVE ELECTRODE-   5 CURRENT COLLECTOR (FOR NEGATIVE ELECTRODE)

1. A particulate active material for a power storage device positiveelectrode, the particulate active material consisting of: particles ofan electrically conductive polymer; and a conductive agent; wherein theparticles of the electrically conductive polymer each have a surfacecoated with the conductive agent.
 2. The particulate active materialaccording to claim 1, wherein the conductive agent is present in aproportion of 1 to 30 parts by weight based on 100 parts by weight ofthe electrically conductive polymer.
 3. The particulate active materialaccording to claim 1, wherein the electrically conductive polymer is apolyaniline or a polyaniline derivative.
 4. The particulate activematerial according to claim 1, wherein the conductive agent is presentin a proportion of 13 to 30 parts by weight based on 100 parts by weightof the electrically conductive polymer.
 5. A power storage devicepositive electrode comprising the particulate active material accordingto claim
 1. 6. The power storage device positive electrode according toclaim 5, wherein the conductive agent is present in a proportion of 1 to30 parts by weight based on 100 parts by weight of the electricallyconductive polymer in the active material.
 7. The power storage devicepositive electrode according to claim 5, wherein the electricallyconductive polymer is a polyaniline or a polyaniline derivative.
 8. Thepower storage device positive electrode according to claim 5, whereinthe conductive agent is present in a proportion of 13 to 30 parts byweight based on 100 parts by weight of the electrically conductivepolymer.
 9. A power storage device comprising: an electrolyte layer; anda positive electrode and a negative electrode provided in opposedrelation with the electrolyte layer interposed therebetween; wherein thepositive electrode comprises the particulate active material accordingto claim
 1. 10. The power storage device according to claim 9, whereinthe conductive agent is present in a proportion of 1 to 30 parts byweight based on 100 parts by weight of the electrically conductivepolymer in the active material.
 11. The power storage device accordingto claim 9, wherein the electrically conductive polymer is a polyanilineor a polyaniline derivative.
 12. The power storage device according toclaim 9, wherein the conductive agent is present in a proportion of 13to 30 parts by weight based on 100 parts by weight of the electricallyconductive polymer.
 13. A method for producing a particulate activematerial for a power storage device positive electrode, the particulateactive material consisting of: particles of an electrically conductivepolymer; and a conductive agent; wherein the particles of theelectrically conductive polymer each have a surface coated with theconductive agent, the method comprising the step of shearing theelectrically conductive polymer particles and the conductive agent bymeans of a composite particle producing apparatus to coat surfaces ofthe electrically conductive polymer particles with the conductive agent.14. The method according to claim 13, wherein the conductive agent ispresent in a proportion of 1 to 30 parts by weight based on 100 parts byweight of the electrically conductive polymer in the active material.15. The method according to claim 13, wherein the electricallyconductive polymer is a polyaniline or a polyaniline derivative.
 16. Themethod according to claim 13, wherein the conductive agent is present ina proportion of 13 to 30 parts by weight based on 100 parts by weight ofthe electrically conductive polymer.